Coatings prepared from aqueous dispersions of polymer particles can be deficient in gloss, resistance properties, and hardness development as compared to those prepared from solvent-based polymers. Dispersions containing a blend of different aqueous polymer dispersions—for example, a blend of acrylic and polyurethane dispersions or a blend of acrylic and epoxy dispersions—are known to improve coating properties; however, blends in the wet state can exhibit storage instability and be solids limited. In addition, coatings derived from these blends can exhibit performance issues due to latex incompatibility and polymer phase demixing.
One approach for improving coatings properties is the use of composite particle technology as described by Guyot et al. in “Hybrid Polymer Latexes” in Prog. Polym. Sci. 32 (2007) 1439-1461. Composite particles contain at least two distinct polymers that may form a homogenous blend within the particle or distinct microphases that are more intimately mixed than physical blends due to their method of preparation. Such composite particles can be prepared, for example, by swelling a dispersion of polymer particles such as polyurethane particles with an ethylenically unsaturated monomer such as acrylic and methacrylic monomers, then polymerizing these monomers to form polyacrylic-polyurethane particles. Alternatively, different types of pre-formed polymers can be co-dispersed into water from either the bulk phase or from an organic solution.
Moreover, monomers can serve a dual role: as a solvent for the preparation of a prepolymer and, subsequently, as a second polymer. For example, acrylic and methacrylic monomers can be used as a solvent to prepare a polyurethane prepolymer, which is subsequently dispersed in water and then chain extended with diamines to build molecular weight. The (meth)acrylic monomers are subsequently polymerized by radical polymerization.
In yet another example of forming composites, a dispersion of polymer particles can be mixed with a monomer or a prepolymer, which is substantially grafted to the first polymer by way of pendant functional groups.
The above-described processes typically result in composites that are either substantially crosslinked and, therefore, are limiting for applications where low temperature film-forming is desired, or that disadvantageously require organic solvents for manufacture or that contain organic solvents in their final dispersion, or are otherwise energy intensive to prepare.
The imbibed polymers are limited to low molecular weight polymers due to viscosity buildup, and the final dispersions are limited to lower potential solids content. A low molecular weight imbibed polymer can negatively impact the properties of the composite materials in its final application, such as a decrease in hardness, scratch resistance and dirt pick-up resistance of a coating made from such a material. It would therefore be advantageous to discover an efficient way to make composites that do not require organic solvents, that have little or no crosslinking, and that have capability for high solids content and high molecular weight.